The present invention relates generally to an optical inspection method and apparatus for inspecting objects such as small components on printed circuit boards, and is particularly directed to a mirror for use in such a method.
The inspection of complex components such as are found on electronic printed circuit boards is tedious and requires careful scrutiny by the operator. In standard inspection systems, the operator scans the top of a circuit board from above using a magnifying viewing system. An inspection apparatus of this type is described in my U.S. Pat. No. 4,379,647, entitled Optical Comparator and Inspection Apparatus.
Normal inspection of a printed circuit board involves looking down on the top of the board. The connecting leads which project from the sides of the components can therefore be inspected. However, in order to save space on the board components are now becoming much smaller with the development of so-called surface mount devices (SMDs), which are mounted very close to the board. The method of connecting these miniaturized components to the board has also been changed. Soldering is accomplished by several methods all designed to make the connection between the SMD components and the circuit board traces on the same side of the board as the component, and to make the connection take up as small an area as possible. In some cases the component is cemented to the circuit board before soldering. In other cases the component is held in place by solder paste for soldering. The solder is generally preprinted on the circuit board as a paste on solder pads on the board. Methods used to melt the preprinted solder include vapor phase, infrared and reflow techniques. However, in all of these techniques problems such as solder spatter, failure to "wick", and thermal cracks can occur, and good visual inspection is required in order to locate such problems. Additionally, some components now employ so-called "J" leads which are tucked under the edge of the component and thus cannot be inspected from above.
Standard inspection techniques for checking printed circuit boards involving looking straight down at the board are thus often insufficient in the latest boards employing surface mount devices, especially with "J" type leads where the soldering cannot be seen by looking straight down. One way of observing such leads is by tilting the the circuit board to try to look under the component. This is only partially successful, particularly where components are packed very close together on the board, and is difficult to do past 30 degrees. Most inspection techniques are of the scanning type, where a board moves under an optical viewing device, and any tilting of the board also makes it difficult for the viewer to keep track of inspected areas. Smaller circuit boards, of 4 or 5 square inches in size, can be turned on edge to inspect at 90 degrees, but this only allows components at the edges of the board to be inspected, since other components will block the view of inner components. Larger printed circuit boards cannot be turned on edge in this way because of the physical restrictions of clearance and focal length of the optical viewing instruments. On the larger PCB's which cannot be tilted to 90 degrees on a standard viewing instrument, even hand tilted methods with an eye loupe are not possible past about 45 degrees. This is usually insufficient to verify the integrity of solder joints.
Large PCB's are often inspected on programmable X-Y tables to permit computerized information on the location of errors. This technique almost eliminates the possibility of tilting the PCB since this would change the inspection direction, and thus the location information would be difficult to track.